This invention relates to a cell for fused-salt electrolysis for reducing alumina in the presence of cryolite to metallic aluminum, comprising an insulated tank, which has a carbon bottom, two rows of anode rods, which are disposed on opposite sides of an anode carrier, which is arranged in the longitudinal center plane, an exhaust gas-collecting duct extending at the longitudinal center between the two rows of anode rods, and two covers, which are downwardly inclined from the outside of respective rows of anode rods to respective longitudinal edges of the tank and which are provided each at its outer longitudinal edge and at each of its transverse edges with a skirt that extends as far as to the respective edge of the tank.
The exhaust gases formed in the electrochemical process of recovering aluminum from alumina contain fluorine, which is a product of the reaction of the cryolite in the electrolytic cell, and finely divided carbon, which released by the carbon anodes. For ecological reasons, it is essential to reduce particularly the fluorine content in the exhaust gases to the utmost minimum before such gases are discharged into the atmosphere so that the environment is not adversely affected.
It is known for this purpose to purify the exhaust air which escapes from the buildings which contain plants for fused-salt electrolysis -- this air contains the exhaust gases -- and then to discharge the air in an ecologically innocuous state. To this end the natural chimmey action in the buildings is utilized to move the air in the buildings, which contains the exhaust gases from the electrolysis, to mechanical means, such as blowers, purifiers or the like, which are mounted on the roofs and serve to discharge the emissions. The purifiers consist essentially of liquid-wetted filters, between which a mist is maintained so that dust particles and gaseous constituents are retained by the filters and by the liquid particles and the purified air is discharged into the atmosphere.
Whereas the mechanical means which are available at present can handle and purify all air in the buildings, they do not decisively improve the conditions within the building from the aspect of work hygiene.
To maintain the concentration of the noxious exhaust gases which escape from the cells for fused-salt electrolysis within tolerable limits, it is known to provide the cells for fused-salt electrolysis with a cover. This cover consists of a collecting duct, which extends longitudinally between the rows of anode rods and which is provided at its outer periphery with two cover plates, which are downwardly inclined toward the longitudinal side of the tank. The anode rods extend through the cover plates and those edge portions of the cover plates which are near the longitudinal sides of the tank closely embrace the anode blocks so that an uncovered surface of the electrolytic cell remains exposed between the inside periphery of the tank and the outside edge of the cover plates and permits of an access to the interior of the cell whereas the exhaust gases from the cell can be sucked off only from a relatively small part thereof.
It is known to eliminate this disadvantage by the use of a cell for fused-salt electrolysis in which the cover plates are mounted on a tube which is longitudinally bisected and which engages the upper portion of the periphery of the cylindrical exhaust gas-collecting duct. Hinged plates are mounted at the outside edges of the cover plates and rest on the longitudinal edges of the tank. The rate at which exhaust gases escape into the air in the building is much reduced by this arrangement.
On the other hand, the arrangement has the disadvantage that these means must be entirely removed for a replacement of the anode.
For this reason it has been proposed (Opened German Specification No. 2,251,898) so to design the cover that the same must be opened only when it is desired to charge the cell and to remove the molten aluminum. This object is accomplished in that the cover covers the anodes on their vertically extending sides, transverse walls intended to rest on the ends of the tank are mounted on the cover, and the anodes consist of continuous anodes. This arrangement is not highly suitable where so-called manipulators are to be used, which can, e.g., automatically break up the slag crust, and feed the alumina to the cell. Besides, the arrangement requires the use of continuous anodes.